The present invention relates to a switching apparatus having an overcurrent or overheating protection function, using an electric power semiconductor element which controls the ON-OFF switching operation of a load, and also detects an overload and moves to an automatic interruption mode. More particularly the invention relates to a switch circuit having a protection function, which is suited to control turning a lamp or motor of a car ON or OFF.
A conventional switch circuit with a protection function, having a system for detecting an over-current supplied to a load and automatically interrupting it is generally known. Examples in which the system is applied using a MOS-FET switch circuit are described in, for example, Japanese Patent Application Laid-open 61261920, Japanese Patent Application Laid-open 62-11916, Japanese Patent Application Laid-open 62-143450, and Japanese Patent Application Laid-Open 63-87128, and Japanese Patent Application Laid-Open 63-170145.
However, if a lamp using a tungsten or other filament as a load is used, when the switch circuit is turned on so as to light the lamp, a sharp surge of starting current flows. The reason is that before the lamp lights, the resistance of the filament (at a comparatively low temperature) is low; therefore, when the lamp lights, current rises sharply, and decreases thereafter, due to an increase in the resistance. Thus, in the conventional switch system with over-current protection, the protection function is triggered by the starting current, and a normal lighting operation cannot be performed. Even if a motor is used as a load, when power is supplied to the motor, a large current surge occurs, so that the protection function of the switch circuit is activated and a normal motor driving operation cannot be performed.
In another system, the temperature of an electric power element (switch circuit) is detected, and when the power element is overheated more than the predetermined temperature, the protection function is performed. A switch circuit having such a protection function is described in, for example, Japanese Patent Application Laid-open 2-135818t Japanese Patent Application Laid-Open 1-181474, and Japanese Patent Application Laid-open 7-221261. In these switch circuits, even in the case of a current surge when the lamp lights or the motor starts , no unnecessary protection function is performed, because the time response for temperature rise is slow; thus, a normal lamp lighting operation or a motor starting operation can be performed. By contrast, if a malfunction involving a locking of the motor occurs, the temperature rises and the switch circuit is interrupted.
However, in a conventional overcurrent protected switch circuit which uses a temperature detection system, even if overheating is detected when an error occurs, and the switch circuit is therefore properly interrupted, there is a problem that an ON-OFF cycle occurs, in which the protection function is recovered due to a reduction in the temperature, and thereafter the temperature rises again, so that the cycle is repeated.